A high pressure alkali metal vapor lamp generally comprises an inner arc tube of ceramic material resistant to the attack of the alkali metal vapor at high temperatures, ordinarily polycrystalline alumina and occasionally monocrystalline alumina (synthetic sapphire), within an outer protective envelope of glass. The arc tube contains the discharge filling or ionizable medium comprising an alkali metal such as sodium, generally as an amalgam with mercury and in a quantity considerably in excess of that vaporized during operation, and an inert gas such as xenon to facilitate starting. The ends of the ceramic tube are sealed by suitable closures affording connection to the thermionic electrodes inside. The outer vitreous envelope is usually provided with a screw base having shell and center contacts to which the electrodes are connected. The inter-envelope space is usually evacuated in order to conserve the heat of the arc tube.
The end closures which have been most widely utilized comprise a metal cap, preferably of niobium whose coefficient of thermal expansion is a fair match for that of alumina, hermetically sealed to the end of the alumina tube by a sealing frit or glass. The electrodes are directly attached to these caps to which external electrical connections are made and which thus also perform the function of inleads. One of the caps has a metal exhaust tube extending through it which is used to exhaust air from the tube and introduce the discharge filling during manufacture. It is then sealed off and serves as an external reservoir for the excess sodium-mercury amalgam which condenses in it because during operation it presents the coldest spot to which the filling of the lamp has access.
Ceramic closures are also widely used and of late they have gained favor for reasons of economy, particularly for small wattage lamps such as lamps of less than 100 watts rating. One design of end seal described in U.S. Pat. No. 3,882,346--McVey, utilizes an alumina ceramic plug sealed in the end of the arc tub and having a central perforation through which extends a lead wire of ceramic-matching metal. The sealing is effected through a glassy sealing composition comprising primarily aluminum oxide and calcium oxide, which melts when the assembly is suitably heated and forms the ceramic-to-ceramic and ceramic-to-metal sealing cement upon cooling. Lamps using such a ceramic closure at one end and a ceramic plug through which extends a niobium exhaust tube at the other end are described in U.S. Pat. No. 4,342,938--Strok. They have been commercially successful and are manufactured in large quantities. Such lamps, sometimes known as single wire seal lamps, are cheaper to make than lamps using metal caps and are substantially equally long-lived.
High pressure sodium lamps using identical ceramic closures at both ends and no exhaust tube are also well known. In their manufacture one of the discharge tube ends is provided with a ceramic closure in the form of an alumina plug through which is sealed an inlead supporting an electrode. The tube (or a batch of such tubes) is then placed in a suitable chamber with the sealed end lowermost, the amalgam put in, and the upper ceramic closure with inlead and electrode is located at the upper end together with sealing frit appropriately distributed to flow into and seal the crevices at the joints when melted. The chamber is first flushed, evacuated, and then filled with the inert gas atmosphere (xenon) desired in the finished lamp. Then while maintaining the lower end cool, the upper end of the tube is heated until the sealing frit flows into the crevices or gaps between alumina plug and tube wall and between plug and inlead. Upon cooling, the arc tube is hermetically sealed and the inert gas pressure in it is of course determined by the pressure in the chamber when the sealing frit solidified. Lamps of this kind using ceramic closures in which a wire inlead is sealed through at each end are sometimes known as double wire seal lamps.
High pressure sodium lamps of the double wire seal kind have been observed to have lives which are much shorter than those of single wire seal lamps, as little as half as long or even less. In such lamps the unvaporized excess of sodium-mercury amalgam, which always finds the coldest spot in the arc tube whereat to condense, generally condenses in the end corners, that is in the right angle where the alumina end plug meets the alumina tube wall. During lamp operation, the amalgam condensed in the end corners lies in contact with and generally covers the internal fillet of sealing frit or glass. It appears that the sealing frit is much less resistant to attack by sodium-mercury liquid condensate than by sodium-mercury vapor at the same temperature. The sealing frit is highly hygroscopic and sensitive to atmospheric impurities. It is theorized that the resistance of the sealing frit to sodium is reduced substantially by the slightest degree of contamination and the reduction is greater when the contact is by liquid sodium than by sodium vapor. The chemical attack on the sealing frit by sodium reduces the ratio of sodium to mercury atoms in the vapor discharge. The result is progressive lamp voltage rise and color shift towards red. Ultimately lamp cycling occurs when the ballast open-circuit voltage can no longer sustain the discharge. Chemical attack can also eventually destroy the hermetic seal and the life of the lamp is prematurely ended.